nutrient deficiency and bleaching -and- Perhaps you needto do a bit more reading ...
lesk at bu.edu
Sat May 12 15:26:15 EDT 2001
Debbie, I suspect you might find that nutrient pulses aid reef recovery
where herbivore densities are still adequate to offset macrophyte growth.
In other words, on a reef thick with macrophytes and/or thin on
herbivorous fishes, urchins, etc. (arthropods and molluscs are "etc."),
macrophytes might flush from the nutrients and cause problems for the
corals. On a well-grazed reef, the corals will get the bennies without
the bullying. It might appear, however, that the removal of the fishes
compromised the resiliency of the reef for reasons other than some of them
being herbivores: i.e., your hypothesis about nutrient limitation.
Of course, even this is a bit simplistic. Some folks think that the
very high densities achieved by Diadema antillarum on some Caribbean
reefs were the result of low predation by fishes (porgies, hogfishes,
and queen triggerfish are similarly delicious to people). Via that
route, overfishing could actually help the corals. Of course, the
urchin plague took care of that for a while in the west Atlantic, but
things are turning around a bit now. You want to see all the
possibilities, and then judge which is the most likely (and worth
actually testing) for any given place or situation. On the whole yes,
reefs are in trouble in many places. If we were to stop overfishing it
would probably help. If we were to stem runoff and eutrophication, that
would probably help too.
Debbie MacKenzie wrote:
> Hi Christine, Eric, coral-list,
> Christine, you wrote:
> >I have some thoughts
> >re your question whether bleaching could be caused by nutrient depletion,
> >however, I am afraid they take Ove's side.
> I would just like to clarify exactly what it is that Ove and I seem to
> disagree on. It certainly is not the obvious fact that the majority of
> coral bleaching events are "thermally induced," or at least are strongly
> associated with times of warmer water. Regarding whether thermally induced
> bleaching "could be caused by nutrient depletion" - my impression was that
> Ove agreed with me that it could, he just thinks it's unlikely. He
> certainly did not claim to have disproved it. So if we disagree, I'm
> thinking it's basically on the importance of this particular idea, and
> whether or not it warrants investigation. Considering the shocking extent
> of the coral bleaching problem and the dire predictions that are being made
> for their future by scientists like Ove, I think that any possibility,
> however remote, ought to be fully investigated.
> >How come that bleaching is usually more severe nearshore, where nutrients
> >are enhanced to levels, which in turn can become detrimental to many coral
> >reef organisms, which are highly adapted to exist in oligotrophic
> >conditions? Could that maybe relate to some patchiness, too: too much
> >'food' and maybe toxic substances?
> It's my impression that "thermal stress" is apt to be higher nearshore. But
> you are right that those kinds of pictures are complicated by the effects
> of multiple stressors, heat, pollution and fishing, so it's very difficult,
> maybe impossible, to pinpoint the exact effects of each. That's why I think
> that their relative impacts will be best sorted out in areas not receiving
> terrestrial runoff. Especially if one wants to isolate the effects of
> fishing/biomass removal alone on the health of corals.
> But before you investigate the effects of too-low "nutrient" levels on
> corals, I think you need to re-examine the meaning of the word "nutrient."
> I discussed this in a fair amount of detail in my (admittedly too long)
> essay ( http://www.fisherycrisis.com/coral1.html ). In the oligotrophic
> waters that are normally found on coral reefs, the absolute level of
> dissolved nutrients found there only represents the limit of the efficiency
> of the organisms in removing them from the water. The nutrient recycling
> patterns on the reefs (and elsewhere in aquatic systems, although to
> varying degrees) conserve the nutrients in solid form and many circular
> routes can be completed without the individual nutrients passing through
> the "dissolved" stage. Fishing removals can therefore result in the effects
> of "nutrient" depletion being felt despite apparently unchanging absolute
> levels of nutrients in dissolved form.
> >From "Life and Death of Coral Reefs" Birkeland (ed), 1997, a snapshot of
> the (underrated in my opinion) "downside" of the food web:
> "Fish feces have been observed to be fed upon by corals (McCloskey and
> Chesher, 1971) and Tovertson (1982) deduced that some fecal material from
> fishes may be eaten and recycled through five fishes before it reaches the
> seafloor to be consumed by corals or other invertebrates." (p 416)
> ....and "corals or other invertebrates" are consumed by reef fish, some
> portion of "nutrients" therefore coming full circle without passing through
> the "liquid" phase. So, your measurements and thinking on "nutrient levels"
> needs to be expanded somehow to reflect the presence or absence of FISH, IMO.
> >From the same source, p 415,
> "On coral reefs...the movements of fishes may cause enough movement of
> nutrients in coral-reef ecosystems to influence the growth of corals (Meyer
> et al 1983), and overfishing can have large-scale ecosystem-level effects."
> Unfortunately, however, chap 10, "Effects of Reef Fishes on Corals and
> Algae" notes that "the role of fish feces fertilizing the reef" represents
> a "potentially important interaction between fishes and reefs" but the
> author omits it from the discussion. Beyond fish feces, ammonia excreted
> from the gills of fish is available for uptake and use by corals.
> One person commented to me off-list: "Ove can tell you that following
> bleaching a good blast of N & P will help stimulate recovery." A "blast
> of N & P" helps corals recover? No surprise...but might that "blast" have
> been given naturally when standing stocks of reef fish were higher? And the
> starved state of the bleached corals is not unexpected since we know that
> they lost their main food-providers when the zooks left -- but it would be
> very interesting to see whether or not a "blast of N & P" given
> prophylactically might help. Could the susceptibility to bleaching be
> lessened in this way? Maybe when the Hotspot program indicates that
> bleaching risk is rising, experimental "blasts" of N & P could be tried
> here and there to see if the availability of these nutrients might prevent
> the expulsion of the zooks in the first place.
> Eric wrote:
> >The patchiness of bleaching was discussed on the list a while back, and
> >stagnant areas due to flow dynamics even around a coral colony can result
> >local conditions that exacerbate bleaching.
> That sounds reasonable, but which feature of "local conditions" is most
> affected by stagnant flow, "nutrient" levels or water temperature? My hunch
> is that still water would be more prone to becoming extremely
> nutrient-depleted rather than extremely warm, but I DO NOT KNOW! Do you?
> >Finally, the web page sort of reads in a sensationalist manner, in my
> >opinion, that I don't think adds to its credibility.
> I realize that, it's because "you can't please all of the people all of the
> time." I have been trying to discuss this issue with scientists, but at the
> same time I try to write so that my fisherman-neighbours and the general
> public just might get interested and be able to plough through an article.
> Sorry, but most of your scientific literature is essentially unavailable to
> them, they just can't read it.
> And Ove chose to rename this discussion "Perhaps you need to do a bit more
> That's OK, of course I will, but I'll never know half of what this group
> knows about corals. But I would like to point out that I MAY have done "a
> bit more reading" than many in this group on the finer points of other
> marine-ecosystems-in-trouble. The declining abundance and stunted growth of
> fish everywhere is very worrisome. In some places the declining
> productivity is blamed on decreased top-to-bottom mixing patterns - yet we
> have a large area in the Northwest Atlantic (Bay of Fundy, Georges Bank)
> that is constantly mixed by TIDAL action - which has not changed - but the
> "productivity" and growth of fish is way down. And the certainty that fish
> were in trouble solely because of changes in water temperature - that's
> appealing, but it's falling apart in a lot of instances. For example, a
> decade ago in Atlantic Canada we had unusually cold water which caused our
> cod to feed poorly and grow very slowly (all cod papers predicted that when
> the water warmed up the fish would feed better and grow more quickly).
> However, in recent years the water has warmed to a point above the
> long-term mean...and growth of cod is still inexplicably dropping. And
> declining growth has been noted in our deep water fish stocks, living down
> on the "slope" where temperature variations are in hundredths of degrees
> rather than tenths...and no-one tries to stretch it far enough to blame the
> slow down of those fish (exploited and unexploited) on water temperature.
> It's recognized as "biomass depletion" in that case - fishing resulting in
> food shortage for marine life.
> I'm concerned that the coral scientists as well will belatedly discover
> that the problems are not solely driven by the effects of changing water
> temperature - I'm convinced that there is an "environmental impact" of
> fishing that's not been recognized, and it's a generalized food shortage.
> What concerns me is not just coral bleaching or coral diseases, but the
> larger diagnostic problem of a whole ocean in trouble. I see a theme, a
> generalized slowing of feeding and growth, and suspect it's because of
> fishing-induced biomass depletion. The possibility of fishing removals
> adversely impacting the "base" of the food web is adamantly denied by
> (most) scientists I've corresponded with regarding the northern
> fish....they try to reassure me with measurements of chlorophyll
> levels....but if the corals turn out to be "hungry" because of
> fishing-induced biomass depletion, it really strengthens my argument. It's
> rather a huge topic though, very hard to pull it all together in one piece
> of work (and I've found that most scientists are very specialized,
> struggling with only one piece of the overall puzzle - that adds to the
> frustration). For a bit more detail on some of the things that I have read,
> and emerging themes that I think I see, check out "The Marine Nutrient
> Cycle" http://www.fisherycrisis.com/motherocean.html
> Thanks for your interest,
> Debbie MacKenzie
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